a. Field of Invention
The invention relates generally to actuators, and, more particularly, to an actuation apparatus and method utilizing electrically driven fluidic pumping for generating large stresses and strains.
b. Description of Related Art
Fluids in a channel can be pumped by the application of an electric field across the channel. The physical mechanism depends on the fluid, the size of the field, the geometry of the channel holding the fluid, and the frequency of the electric field. Among the many mechanisms are electro-osmosis (EO), electro-hydro-dynamic (EHD) pumping, and dielectrophoresis (DEP). These will be introduced briefly below. The invention can make use of any electric-field driven pumping mechanism, although electro-osmosis is preferred.
Electro-Osmosis This technique is used to pump fluids that contain some quantity of charged species, such as positive and negative ions produced by dissolving salt in a liquid. It is known in the art that most solid surfaces acquire a surface electric charge when brought into contact with a liquid. The surface charging mechanisms include, for example, ionization, ion adsorption, and ion dissolution. These mechanisms occur naturally, and their strength depends upon both the solid and liquid materials, as well as the surface preparation. The surface charge attracts counter-ions to the solid/liquid interface, thus creating a thin “Debye” electric double-layer. When an electric field is applied across the liquid, the ions in the Debye layer migrate in the field, dragging the remainder of the fluid with them by means of viscous forces. This effect is referred to as electroosmosic pumping, or EOF pumping, and is a phenomena that is well known in the art of microfluidics for pumping liquids, typically for transporting liquids in micro-scale applications where the size of this effect is significant due to the large surface to volume ratio.
Specifically, referring to related-art FIG. 1, electroosmotic pumping is illustrated for a channel 10. As shown, ions 12 in liquid 14, shown as (+) charges, collect at interface 15 between the channel wall and the liquid to cancel the (in this example negative) solid surface charges 16. These charges that collect at the interface are referred to as counter-ions, and the opposite charges in fluid 22 are referred to a co-ions. An applied electric field 18 causes the positive counter-ions to move in direction 20, along the electric field, and thus drag along the fluid in the channel by viscous forces. In the center of the channel, there remain almost as many counter-ions as there are oppositely charged co-ions.
The applied field can be either DC (direct current, or constant) or AC (alternating current, or oscillating). Most EO pumping is done at DC, but when high voltages are applied, AC frequencies are advantageous because they reduce or eliminate electrolysis. However, AC pumping is more complex and is frequency-dependent.
In EO pumping, the ratio between the surface forces and the bulk (or volume) forces is important. In general, the higher the surface to volume ratio, the greater the pumping force (provided that the dimensions are not so small that the Debye layers of the walls overlap). Channels can be of arbitrary shape, having cross-sections that are round, oval, square, rectangular, etc. To increase the amount of fluid that can be pumped (and thus increase the actuator speed), while maintaining surface to volume ratios that give high pressure, the number of channels between the chambers can be increased. In addition, one dimension of the channel can be increased. For example, a narrow but deep channel can be used, since the small width dimension will ensure a high surface area to volume ratio. In addition, the channel can be filled with a porous or gel medium, creating, in effect, many parallel convoluted channels.
Electro-Hydro-Dynamic Pumping This technique, also known as ion-drag pumping, can be used to pump dielectric fluids (i.e. ones that do not conduct a current), which do not inherently contain charged species. A very high electric field is used, high enough to inject electrons from the electrodes into the fluid. The injected electrons ionize fluid molecules, making them negatively charged. These ions move toward the positively charged electrode, dragging other fluid molecules with them.
Dielectrophoresis When an electric field is applied to a polarizable particle or molecule, it develops a dipole (plus charge on one side and minus on the other). The dipole can be moved in the field if the field is asymmetric (if it is symmetric, the forces on the plus and minus simply cancel). DEP uses AC voltages. DEP effects will appear if the channel is not of uniform cross-section, or if there are changes in the material making up the channel, or through other effects that cause non-homogeneous fields.
Conventional actuators, such as motors and hydraulic actuators, are in wide use and can provide large forces and displacements. However, they are rigid, discrete units made of discrete parts. In some applications, it would be more advantageous to have actuators that are more like biological muscles, which expand and contract, come in a range of sizes and shapes, and can be conformally placed on other structures, like the muscles of the face are placed over the skull. It is of particular interest to have these artificial muscle-like materials be electrically controlled (rather than, for example, chemically controlled). Such actuators made from existing electroactive materials include piezo-electric ceramics and polymers, shape memory alloys (SMA), and dielectric elastomer actuators (DEA). Neither the conventional actuators nor the electroactive materials employ electroosmotic pumping for generation of stress and/or strain. In addition, conventional actuators and some electroactive materials cease to function if a small portion of their structure is damaged.
For all actuators, two important performance parameters are stress (force per unit area) and strain (change in length divided by the original length). Among the electroactive materials, those that are stiff (have a high Young's modulus) provide higher forces, but those that are soft can undergo large displacements.
It would therefore be of benefit to provide an actuation apparatus and method utilizing electro-osmotic pumping for generating stresses and strains that are capable of exceeding those generated by existing muscle-like actuators. It would also be of benefit to provide an actuation apparatus and method which can be produced in a variety of shapes and sizes, and either used as a stand-alone structural material or be applied over and/or within another material or structure. It would be of benefit to provide an actuation apparatus and method that can continue to function if part of the structure is damaged. It would also be of benefit to provide an actuation apparatus and method which would be applicable in a variety of fields, such as mechanics, aerodynamics etc., and an actuation apparatus and method which is robust in design, and which is relatively simple and economical to manufacture and implement.
Definitions
Stress is force per unit area and has units of Pascals.
Strain is the change in length divided by the original length, or ΔL/L, and has no units; it is often given in percent.
Actuating cell means a single microfluidic pumping unit consisting of at least two chambers, a connecting channel, and electrodes. These shall also be referred to these as building blocks and unit elements.
Fluidic actuator material refers to a device consisting of a group of actuating cells that have been configured in a predetermined way.
Substrate means the material into which the chambers and channels are fabricated.